Growing onions indoors is entirely achievable, especially for those interested in a steady harvest of mild, flavorful onion greens. While raising large, mature onion bulbs indoors presents a significant challenge due to light and space limitations, cultivating green onions, or scallions, is straightforward. This makes indoor onion growing a rewarding project for any gardener seeking a continuous supply of fresh produce.
Selecting the Best Onion Variety for Indoors
Success with indoor onion growing depends heavily on choosing a variety suited to your goals. The two main types of cultivation are for the green tops (scallions or bunching onions) and for the large, subterranean bulbs. Growing large bulbing onions indoors is impractical because they require a long period of intense light exposure—often 14 to 16 hours a day for long-day varieties—to trigger bulb formation.
For the indoor environment, the best selection is a bunching onion variety, such as a Japanese bunching onion, which is grown for its continuous green shoots and does not form a large bulb. These types, along with growing from onion sets (small, immature bulbs) or kitchen scraps, offer the quickest and most reliable yield of fresh greens. Choosing “cut-and-come-again” varieties ensures you can sustain a harvest for an extended period, which is the primary benefit of indoor cultivation.
Setting Up the Ideal Indoor Growing Environment
Onions thrive in a cool, bright environment. Lighting is the most important factor for an indoor crop, and a south-facing window is typically insufficient during darker months. Supplemental lighting from a full-spectrum grow light is recommended for 10 to 12 hours daily to ensure strong, upright growth. The light fixture should be positioned about four to six inches above the onion greens to maximize intensity.
Maintaining a cool temperature between 60°F and 72°F (15°C to 22°C) promotes healthy growth. The container must have excellent drainage to prevent root rot, as onions dislike sitting in soggy soil. A container at least six to eight inches deep is adequate for growing greens from sets or scraps. It should be filled with a loose, well-draining potting mix, often enriched with compost. The soil should remain consistently moist but never waterlogged.
Planting and Ongoing Maintenance Instructions
Onions can be started from seeds, small bulbs called sets, or by regrowing the root end of a store-bought scallion. Starting from a set or a scrap is the easiest method for a quick harvest of greens.
Planting Sets and Scraps
When planting sets, place the small bulb one to two inches deep in the soil with the pointed tip visible above the surface, spacing them about two inches apart. For the kitchen scrap method, plant the bottom inch of the white end of a scallion into the soil, ensuring the roots are covered.
Watering and Fertilizing
Consistent moisture is crucial for onions, which have shallow root systems. Check the soil daily, watering thoroughly until it drains from the bottom, and allow the top layer to dry slightly before the next watering. Onions are heavy feeders, so a light application of a balanced liquid fertilizer every four to six weeks will support continuous leaf production. Good air circulation, often achieved by placing the container near a window or small fan, helps to minimize the risk of fungal issues and deter pests.
Maximizing Your Indoor Onion Harvest
The most rewarding aspect of growing green onions indoors is the continuous harvesting method, often called “cut-and-come-again.” Once the green shoots reach a usable height of six to eight inches, they are ready to be clipped. To ensure regrowth, use clean scissors to cut the green stalk about one inch above the soil line, leaving the white bulb and root system intact. The remaining base will produce new green shoots within a week or two, providing an ongoing supply.
If you attempt to grow bulbing onions, the harvest timeline is much longer, generally requiring three to five months. Maturity is signaled when the tops begin to yellow and fall over naturally. At this stage, stop watering and allow the bulbs to cure in a dry, well-ventilated location for several weeks before storage. Focusing on the continuous harvest of fresh green onions delivers the most practical and consistent yield for most indoor setups.